There are many circumstances in which it is desirable to prevent vibrations in one structure or member from being transmitted to an adjacent structure or member. Such vibration isolation is often desirable, for example, in industrial machinery. In a drop forge, a press, or mechanical power transmission equipment, for example, vibrations are set up by the action of the moveable components of the machinery. The vibrations are transmitted to other components in the machinery and to nearby structures, such as the floor on which the machinery is supported. High levels of transmitted vibration tend to increase noise levels in work areas, to cause damage to sensitive recording and other instrumentation, and to interfere with the proper operation of machinery.
In order to prevent the transmission of vibrations from machinery, for example, to adjacent structures, resilient mountings or supports are commonly used to mount the machinery on a floor or other supporting surface. Resilient mountings or supports may also be provided within the machinery to isolate the moving components, which represent a source of vibrations, from other parts of the machinery. When the individual mountings are simple springs, such as bodies of elastomer, the mountings, at low frequencies of vibration, transmit essentially all of the vibrational forces to adjacent and/or supporting structures. As the frequency of vibration begins to increase, the mountings typically transmit to the adjacent and/or supporting structures forces which are greater than the vibrational forces acting on the supported member or component. The increase in the force transmitted, as compared to the force acting on the supported member, peaks at what is known as a "natural" frequency of vibration of the system that comprises the supported member, the resilient mounting or support, and the supporting member or structure. Beyond the natural frequency, the ratio of force transmitted to the force on the supported member decreases to less than one. Thus, at higher frequencies of vibration, the forces transmitted to the supporting member or structure are substantially less than the forces acting on the supported member. The range of frequencies at which transmitted force is less than the force on the supported member is known as the isolation range of the resilient mounting.
In the isolation range of vibrational frequencies of a simple or one-stage resilient mounting or support, the percentage of vibrational force that is transmitted to an adjacent and/or supporting structure or member is, to some extent, a function of the stiffness of the mounting. Thus, if the mounting is fabricated of a less stiff or more resilient material, a smaller percentage of the exciting force will be transmitted from the supported structure or member to the supporting structure or member at any given frequency in the isolation range of the mounting. At the same time, however, the deflection experienced by a mounting in response to any given force is also affected by the resilience of the mounting. Thus, as the stiffness of the mounting decreases, the supported member moves through a greater distance in response to a predetermined force. With respect to machinery, large movements between adjacent structures often cannot be accommodated. Within a machine, the space between any two components may be limited. Between the machine and the floor of a factory, for example, large movements of the machine may be possible, yet the movements may make operation of the machine substantially more difficult, if not impossible. Unacceptably large movements may result either from dynamic loads or from static loads such as the weight of the machinery.
Especially in situations where movements and deflections are critical, another approach may be utilized to reduce the percentage of force that is transmitted from the supported structure or member to the supporting structure or member or, stated another way, to reduce the transmissibility of a mounting. More specifically, the mounting is constructed to incorporate two resilient elements separated by an intermediate element that has a mass of relative significance (e.g. at least 5% of the mass of the supported member). With such a "two-stage" isolator that incorporates an intermediate mass or mass body, vibrational forces from the supported structure or member must pass through both of the resilient elements and through the intermediate mass before reaching the supporting structure or member. The vibrational forces will excite the intermediate mass element to produce another natural frequency of the system that comprises the supported and supporting members and the mounting. The natural frequency that is determined primarily by the intermediate mass will typically occur at a higher frequency than the natural frequency that results from resiliently mounting the supported member on a simple, one-stage mounting. Consequently, the isolation range of a two-stage mounting begins at a higher frequency than the isolation range of a similar single stage mounting. On the other hand, the decrease in the transmissibility of the two-stage mounting in its isolation range is more rapid and of a greater magnitude than the decrease in transmissibility afforded by a single-stage mounting. The basic advantages of a two-stage or intermediate mass type mounting have previously been recognized, as is illustrated by Whitehill U.S. Pat. No. 3,314,631, assigned to the assignee of the present invention, and Young et al U.S. Pat. No. 3,764,100.
The Whitehill and Young et al patents describe and illustrate two different two-stage or intermediate mass type resilient mountings or supports that may be utilized to reduce the transmission of vibrational forces from a supported structure or member to a supporting structure or member where the forces produce translational (e.g. vertical or lateral) movements of the supported member. The Whitehill patent also recognizes that the vibrational forces acting on a supported member may include forces that tend to produce tilting or cocking motions of the supported member. To suppress the tilting or cocking modes of vibration, Whitehill proposes to utilize several two-stage resilient mountings. Each mounting is located a substantial distance laterally from the center of gravity of the supported member and, thus, has a substantial moment arm for resisting tilting movements of the supported member. Whitehill also suggests that the mountings be located so as to act substantially in a horizontal plane through the center of gravity of the supported member. Consequently, while the Whitehall patent offers some suggestions on how to isolate vibrations that produce tilting or cocking movements of a supported member, the patent does not offer any solutions for situations in which mountings can not be placed either so as to act through the center of gravity of the supported member or a substantial distance laterally from the center of gravity of the supported member.
A textile spindle is one example of a machinery element which is subject to vibrational forces that cannot be isolated from the supporting structure in accordance with the teachings of the Whitehill patent. A textile spindle must be mounted near one of its ends in order that its other end may be free to accept the package of yarn which is being wound on the spindle. Consequently, a vibration isolating mounting for the spindle can not be arranged to act in a horizontal plane through the center of gravity of the spindle. In addition, textile spindles are typically mounted through openings formed only a few inches apart in a long supporting rail. The limited space between adjacent spindles, which permits maximum production in a minimum of space, also prohibits, as a practical matter, using several two-stage mountings spaced at distances from the spindle sufficiently large to produce moment arms that will resist tilting movements of the spindle.